Impeller shroud with slidable coupling for clearance control in a centrifugal compressor

ABSTRACT

A system for controlling the clearance distance between an impeller blade tip of a centrifugal compressor and a radially inner surface of an impeller shroud in a turbine engine. The system comprises a high pressure air source, an air piston mounted between an engine casing and the shroud and adapted to receive high pressure air from the high pressure air source, a mounting arm coupling the shroud and air piston, and a slidable coupling adapted to allow axial movement of the shroud and joining the shroud to an axial member.

RELATED APPLICATIONS

The present application is a divisional application of and claimspriority to U.S. patent application Ser. No. 15/234,601, now U.S. Pat.No. ______, filed Aug. 11, 2016, which is a continuation in part of andclaims priority to U.S. patent application Ser. No. 15/165,404, now U.S.Pat. No. 10,352,329, filed May 26, 2016; U.S. patent application Ser.No. 15/165,468, now U.S. Pat. No. 10,309,409, filed May 26, 2016; andU.S. patent application Ser. No. 15/165,728, now U.S. Pat. No.10,408,226, filed May 26, 2016, the entirety of which are herebyincorporated by reference.

FIELD OF THE DISCLOSURE

The present invention relates generally to turbine engines havingcentrifugal compressors and, more specifically, to control of clearancesbetween an impeller and a shroud of a centrifugal compressor.

BACKGROUND

Centrifugal compressors are used in turbine machines such as gas turbineengines to provide high pressure working fluid to a combustor. In someturbine machines, centrifugal compressors are used as the final stage ina multi-stage high-pressure gas generator.

FIG. 1 is a schematic and sectional view of a centrifugal compressorsystem 100 in a gas turbine engine. One of a plurality of centrifugalcompressor blades 112 is illustrated. As blade 112 rotates, it receivesworking fluid at a first pressure and ejects working fluid at a secondpressure which is higher than first pressure. The radially-outwardsurface of each of the plurality of compressor blades 112 comprises acompressor blade tip 113.

An annular shroud 120 encases the plurality of blades 112 of theimpeller. The gap between a radially inner surface 122 of shroud 120 andthe impeller blade tips 113 is the blade tip clearance 140 or clearancegap. Shroud 120 may be coupled to a portion of the engine casing 131directly or via a first mounting flange 133 and second mounting flange135.

Gas turbine engines having centrifugal compressor systems 100 such asthat illustrated in FIG. 1 typically have a blade tip clearance 140between the blade tips 113 and the shroud 120 set such that a rubbetween the blade tips 113 and the shroud 120 will not occur at theoperating conditions that cause the highest clearance closure. A rub isany impingement of the blade tips 113 on the shroud 120. However,setting the blade tip clearance 140 to avoid blade 112 impingement onthe shroud 120 during the highest clearance closure transient may resultin a less efficient centrifugal compressor because working fluid is ableto flow between the blades 112 and shroud 120 thus bypassing the blades112 by flowing through gap 140. This working fluid constitutes leakage.In the centrifugal compressor system 100 of FIG. 1, blade tip clearances140 cannot be adjusted because shroud 120 is rigidly mounted to theengine casing 131.

It is known in the art to dynamically change blade tip clearance 140 toreduce leakage of a working fluid around the blade tips 113. Severalactuation systems for adjusting blade tip clearance 140 during engineoperation have been developed. These systems often include complicatedlinkages, contribute significant weight, and/or require a significantamount of power to operate. Thus, there continues to be a demand foradvancements in blade clearance technology to minimize blade tipclearance 140 while avoiding rubs.

The present application discloses one or more of the features recited inthe appended claims and/or the following features which, alone or in anycombination, may comprise patentable subject matter.

SUMMARY

According to an aspect of the present disclosure, a compressor shroudassembly is disclosed in a turbine engine having a dynamically moveableimpeller shroud for encasing a rotatable centrifugal compressor andmaintaining a clearance gap between the shroud and the rotatablecentrifugal compressor. The assembly comprises a static compressorcasing; an actuator mounted to the casing; and an impeller shroudslidably coupled at a forward end to the casing and mounted proximate anaft end to the actuator, the impeller shroud moving relative to therotatable centrifugal compressor in an axial direction whilesubstantially maintaining a radial alignment when the actuator isactuated.

In some embodiments the slidable coupling between the shroud and thecasing is dimensioned to maintain an air boundary during the full rangeof axial movement of the shroud. In some embodiments the assemblyfurther comprises one or more sensors for measuring the clearance gapbetween the shroud and the rotatable centrifugal compressor, theactuator being actuated or vented in response to the clearance gapmeasure by the one or more sensors. In some embodiments the assemblyfurther comprises one or more sensors for measuring discharge pressureof the rotatable centrifugal compressor, the actuator being activated inresponse to the measured pressure.

In some embodiments the actuator is one of a pneumatic, hydraulic,electric, or thermal actuator. In some embodiments the actuator is anair piston comprising a chamber adapted to receive actuating air and anaft extending mounting arm which moves axially substantially maintaininga radial alignment when the piston is actuated. In some embodiments theactuator is a thermal actuator comprising one or more linkage assembliesmounted to the casing and being spaced around the circumference thereof;and an annular thermal driver mounted to the linkage assemblies. In someembodiments the actuator comprises a driving member extending along aradius of and being rotatable about the axis of rotation of therotatable centrifugal compressor, and a driving mechanism coupled to thedriving member to rotate the driving member about the axis of rotationwhen the actuator is activated.

According to another aspect of the present disclosure, a compressorshroud assembly is disclosed in a turbine engine having an impellershroud for encasing a rotatable centrifugal compressor and maintaining aclearance gap between the shroud and the rotatable centrifugalcompressor. The assembly comprises a static compressor casing; anactuator mounted to the casing; and an impeller shroud comprising astatic inducer portion coupled to the casing and a dynamically moveableexducer portion slidably coupled at a forward end to the inducer portionand mounted proximate an aft end to the actuator, the exducer portionmoving relative to the rotatable centrifugal compressor in an axialdirection while substantially maintaining a radial alignment when theactuator is actuated.

According to yet another aspect of the present disclosure, a method isdisclosed of dynamically changing a clearance gap between a rotatablecentrifugal compressor and a shroud encasing the rotatable centrifugalcompressor. The method comprises mounting an actuator to a staticcasing; mounting a shroud to the actuator; slidably coupling a forwardend of the shroud to the casing; and actuating the actuator to therebymove the shroud relative to a rotatable centrifugal compressor, theshroud moving relative to the rotatable centrifugal compressor in anaxial direction while substantially maintaining a radial alignment.

In some embodiments the method further comprises sensing the clearancegap between the rotatable centrifugal compressor and the shroud andactuating the actuator in response to the sensed clearance gap. In someembodiments the method further comprises sensing the discharge pressureof the rotatable centrifugal compressor and actuating the actuator inresponse to the sensed discharge pressure. In some embodiments the stepof actuating the actuator comprises supplying or discharging highpressure air to an air piston to effect axial motion while substantiallymaintaining a radial alignment of the shroud.

BRIEF DESCRIPTION OF THE DRAWINGS

The following will be apparent from elements of the figures, which areprovided for illustrative purposes and are not necessarily to scale.

FIG. 1 is a schematic and sectional view of a centrifugal compressorsystem in a gas turbine engine.

FIG. 2A is a schematic and sectional view of a centrifugal compressorsystem having a clearance control system in accordance with someembodiments of the present disclosure.

FIG. 2B is an enlarged schematic and sectional view of the clearancecontrol system illustrated in FIG. 2A, in accordance with someembodiments of the present disclosure.

FIG. 3A is a schematic and sectional view of a centrifugal compressorsystem having a clearance control system with an air piston inaccordance with some embodiments of the present disclosure.

FIG. 3B is an enlarged schematic and sectional view of the clearancecontrol system with an air piston illustrated in FIG. 3A, in accordancewith some embodiments of the present disclosure.

FIG. 4 is a schematic and sectional view of a clearance control systemwith a bellows-type air piston in accordance with the presentdisclosure.

FIG. 5 is a schematic and sectional view of the pressure regions of aclearance control system with an air piston in accordance with someembodiments of the present disclosure.

FIG. 6A is a schematic and sectional view of a centrifugal compressorsystem having a clearance control system in accordance with someembodiments of the present disclosure.

FIG. 6B is an enlarged schematic and sectional view of the clearancecontrol system illustrated in FIG. 6A, in accordance with someembodiments of the present disclosure.

FIG. 7 is a schematic and sectional view of another embodiment of aclearance control system in accordance with the present disclosure.

FIG. 8 is a schematic and sectional view of the pressure regions of aclearance control system in accordance with some embodiments of thepresent disclosure.

FIG. 9 is a schematic and sectional view of another embodiment of aclearance control system in accordance with the present disclosure.

FIG. 10 is a schematic and sectional view a clearance control system inaccordance with some embodiments of the present disclosure.

FIG. 11 is a schematic and axial view of a plurality of driver armscircumferentially disposed about an impeller shroud in accordance withsome embodiments of the present disclosure.

FIG. 12A is a schematic and sectional view of a centrifugal compressorsystem having a clearance control system is accordance with someembodiments of the present disclosure.

FIG. 12B is an enlarged schematic and sectional view of the clearancecontrol system illustrated in FIG. 12A, in accordance with someembodiments of the present disclosure.

While the present disclosure is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that the present disclosure is notintended to be limited to the particular forms disclosed. Rather, thepresent disclosure is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the disclosure asdefined by the appended claims.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to a number of illustrativeembodiments illustrated in the drawings and specific language will beused to describe the same.

This disclosure presents embodiments to overcome the aforementioneddeficiencies in clearance control systems and methods. Morespecifically, the present disclosure is directed to a system forclearance control of blade tip clearance which avoids the complicatedlinkages, significant weight penalties, and/or significant powerrequirements of prior art systems. The present disclosure is directed toa system which actuates an actuator to cause axial deflection of animpeller shroud. The impeller shroud is slidably coupled, or coupledwith a sliding joint, to an engine casing to form a tight clearancejoint, thus allowing the impeller shroud to axially deflect as a resultof actuating the actuator.

FIG. 2A is a schematic and sectional view of a centrifugal compressorsystem 200 having a clearance control system 260 in accordance with someembodiments of the present disclosure. Centrifugal compressor system 200comprises centrifugal compressor 210 and clearance control system 260.

The centrifugal compressor 210 comprises an annular impeller 211 havinga plurality of centrifugal compressor blades 212 extending radially fromthe impeller 211. The impeller 211 is coupled to a disc rotor 214 whichis in turn coupled to a shaft 216. Shaft 216 is rotatably supported byat least forward and aft shaft bearings (not shown) and may rotate athigh speeds. The radially-outward surface of each of the compressorblades 212 constitutes a compressor blade tip 213.

As blade 212 rotates, it receives working fluid at an inlet pressure andejects working fluid at a discharge pressure which is higher than theinlet pressure. Working fluid (e.g. air in a gas turbine engine) istypically discharged from a multi-stage axial compressor (not shown)prior to entering the centrifugal compressor 210. Arrows A illustratethe flow of working fluid through the centrifugal compressor 210.Working fluid enters the centrifugal compressor 210 from an axiallyforward position 253 at an inlet pressure. Working fluid exits thecentrifugal compressor 210 at an axially aft and radially outwardposition 255 at a discharge pressure which is higher than inletpressure.

Working fluid exiting the centrifugal compressor 210 passes through adiffusing region 250 and then through a deswirl cascade 252 prior toentering a combustion chamber (not shown). In the combustion chamber,the high pressure working fluid is mixed with fuel and ignited, creatingcombustion gases that flow through a turbine (not shown) for workextraction.

In one embodiment, the clearance control system 260 comprises anactuator 299, an annular shroud 220, and a slidable coupling 266.Clearance control system 260 can also be referred to as a compressorshroud assembly.

Actuator 299 is adapted to cause axial movement of annular shroud 220while substantially maintaining radial alignment of annular shroud 220.Actuator 299 is coupled between a portion of engine casing 231 andshroud 220. A forward-extending arm 276 extends axially forward fromactuator 299 and is coupled to engine casing 231 at first mountingflange 233, thus mounting actuator 299 to the casing 231. Anaft-extending arm 277 extends axially aft from actuator 299 and iscoupled to a mounting arm 278 extending axially forward from shroud 220.Aft-extending arm 277 and mounting arm 278 are coupled at mountingflange 237.

As discussed below, in some embodiments actuator 299 may comprise apneumatic piston, a set of thermally-actuated linkages, or a threadedmember.

Shroud 220 is a dynamically moveable impeller shroud. Shroud 220 encasesthe plurality of blades 212 of the centrifugal compressor 210. Shroud220 comprises a forward end portion 223 terminating at slidable coupling266, a central portion 224, and a aft end portion 225. In someembodiments, surface 222 of shroud 220 comprises an abradable surface.In some embodiments, a replaceable cover is provided which covers thesurface 222 and is replaced during engine maintenance due to impingementof blade tips 213 against surface 222.

In some embodiments aft end portion 225 is defined as the radiallyoutward most third of shroud 220. In other embodiments aft end portion225 is defined as the radially outward most quarter of shroud 220. Instill further embodiments aft end portion 225 is defined as the radiallyoutward most tenth of shroud 220. In embodiments wherein mounting arm278 extends axially forward from aft end portion 225, these variousdefinitions of aft end portion 225 as either the final third, quarter,or tenth of shroud 220 provide for the various radial placements ofmounting arm 278 relative to shroud 220.

Slidable coupling 266 comprises an axial member 280 coupled to forwardend portion 223 of shroud 220. Slidable coupling 266 is adapted to allowsliding displacement or translation between axial member 280 and forwardend portion 223. In some embodiments one or more surfaces of forward endportion 223 and/or axial member 280 comprise a lubricating surface toreduce friction and wear between these components. In some embodimentsthe lubricating surface is a coating. In some embodiments slidablecoupling 266 is a tight clearance joint between forward end portion 223and axial member 280. In some embodiments slidable coupling 266 providesa radially centering feature for the shroud 220 relative to the enginecasing 231 and to the engine centerline.

Clearance control system 260 is coupled to the engine casing 231 via afirst mounting flange 233 and second mounting flange 235. In someembodiments engine casing 231 is at least a portion of a casing aroundthe multi-stage axial compressor.

The gap between a surface 222 of shroud 220 which faces the impeller 211and the impeller blade tips 213 is the blade tip clearance 240. Inoperation, thermal, mechanical, and pressure forces act on the variouscomponents of the centrifugal compressor system 200 causing variation inthe blade tip clearance 240. For most operating conditions, the bladetip clearance 240 is larger than desirable for the most efficientoperation of the centrifugal compressor 210. These relatively largeclearances 240 avoid rubbing between blade tip 213 and the surface 222of shroud 220, but also result in high leakage rates of working fluidpast the impeller 211. It is therefore desirable to control the bladetip clearance 240 over a wide range of steady state and transientoperating conditions. The disclosed clearance control system 260provides blade tip clearance 240 control by positioning shroud 220relative to blade tips 213.

FIG. 2B is an enlarged schematic and sectional view of the clearancecontrol system 260 illustrated in FIG. 2A, in accordance with someembodiments of the present disclosure. The operation of clearancecontrol system 260 will be discussed with reference to FIG. 2B.

In some embodiments during operation of centrifugal compressor 210 bladetip clearance 240 is monitored by periodic or continuous measurement ofthe distance between surface 222 and blade tips 213 using a sensor orsensors positioned at selected points along the length of surface 222.When clearance 240 is larger than a predetermined threshold, it may bedesirable to reduce the clearance 240 to prevent leakage and thusimprove centrifugal compressor efficiency. Actuator 299 may be actuatedbased on measured blade tip clearance 240 to move shroud 220 and thusadjust the blade tip clearance 240 as desired.

In other embodiments, engine testing may be performed to determine bladetip clearance 240 for various operating parameters and actuator 299schedule is developed for different modes of operation. For example,based on clearance 240 testing, actuator 299 may actuate a predetermineddegree for cold engine start-up, warm engine start-up, steady stateoperation, and max power operation conditions. As another example, atable may be created based on blade tip clearance 240 testing, andactuator 299 actuation is adjusted according to operating temperaturesand pressures of the centrifugal compressor 210. A sensor may be used tomonitor the degree of actuation of actuator 299. Thus, based onmonitoring the operating conditions of the centrifugal compressor 210such as inlet pressure, discharge pressure, and/or working fluidtemperature, a desired blade tip clearance 240 is achieved according toa predetermined schedule of actuation for actuator 299.

Regardless of whether clearance 240 is actively monitored or controlledvia a schedule, in some operating conditions it may be desirable toreduce the clearance in order to reduce leakage past the centrifugalcompressor 210. In order to reduce the clearance 240, actuator 299 isactuated to cause movement in an axial direction. With actuator 299rigidly coupled, or “grounded”, to casing 231 via forward-extending arm276, actuation is enabled in the axially aft direction as indicated byarrow 291 in FIG. 2B.

The axially aft motion displaces aft-extending arm 277 and mounting arm278. Mounting arm 278 is coupled to and imparts a force on the aft endportion 225 of shroud 220, thus moving the aft end portion 225 in anaxially aft direction as indicated by arrow 292. This movement of aftend portion 225 is translated to a similar axially aft movement at theslidable coupling 266, where forward end portion 223 is displaced in anaxially aft direction relative to axial member 280 as indicated by arrow293. Shroud 220 thus moves relative to the centrifugal compressor 210 inan axial direction while substantially maintaining the radial alignmentof shroud 220.

The axially aft movement of shroud 220 caused by actuator 299 actuationresults in shroud 220 moving closer to blade tips 213, thus reducing theclearance 240 and leakage. During many operating conditions thisdeflection of shroud 220 in the direction of blade tips 213 is desirableto reduce leakage and increase compressor efficiency.

Where monitoring of blade tip clearance 240 indicates the need for anincrease in the clearance 240, actuator 299 is actuated to cause axiallyforward movement of aft-extending arm 277, mounting arm 278, and aft endportion 225. The axially forward movement of aft end portion 225 resultsin similar movement of shroud 220, including the sliding displacement inan axially forward direction of forward end portion 223 against axialmember 280. Thus, by actuating the actuator 299, shroud 220 is movedaxially forward, away from blade tips 213 and increasing blade tipclearance 240.

Slidable coupling 266 is dimensioned such that an air boundary ismaintained through the full range of axial movement of shroud 220.

FIG. 3A is a schematic and sectional view of a centrifugal compressorsystem 1200 having a clearance control system 1260 in accordance withsome embodiments of the present disclosure. Centrifugal compressorsystem 1200 comprises centrifugal compressor 1210 and clearance controlsystem 1260.

The centrifugal compressor 1210 comprises an annular impeller 1211having a plurality of centrifugal compressor blades 1212 extendingradially from the impeller 1211. The impeller 1211 is coupled to a discrotor 1214 which is in turn coupled to a shaft 1216. Shaft 1216 isrotatably supported by at least forward and aft shaft bearings (notshown) and may rotate at high speeds. The radially-outward surface ofeach of the compressor blades 1212 constitutes a compressor blade tip1213.

As blade 1212 rotates, it receives working fluid at an inlet pressureand ejects working fluid at a discharge pressure which is higher thanthe inlet pressure. Working fluid (e.g. air in a gas turbine engine) istypically discharged from a multi-stage axial compressor (not shown)prior to entering the centrifugal compressor 1210. Arrows A illustratethe flow of working fluid through the centrifugal compressor 1210.Working fluid enters the centrifugal compressor 1210 from an axiallyforward position 1253 at an inlet pressure. Working fluid exits thecentrifugal compressor 1210 at an axially aft and radially outwardposition 1255 at a discharge pressure which is higher than inletpressure.

Working fluid exiting the centrifugal compressor 1210 passes through adiffusing region 1250 and then through a deswirl cascade 1252 prior toentering a combustion chamber (not shown). In the combustion chamber,the high pressure working fluid is mixed with fuel and ignited, creatingcombustion gases that flow through a turbine (not shown) for workextraction.

In one embodiment, the clearance control system 1260 comprises anactuator 299, an annular shroud 1220, and a slidable coupling 1266.Clearance control system 1260 can also be referred to as a compressorshroud assembly. In some embodiments, actuator 299 may comprise highpressure air source 1262 and an air piston 1264.

High pressure air source 1262 provides high pressure actuating air toair piston 1264. In some embodiments high pressure air source 1262 issupplied from centrifugal compressor discharge air.

Air piston 1264 is adapted to receive high pressure air from highpressure air source 1262. Air piston 1264 comprises a forward rigidmember 1271, aft rigid member 1272, and a central flex member 1273disposed between forward rigid member 1271 and aft rigid member 1272.Together, forward rigid member 1271, aft rigid member 1272, and centralflex member 1273 define a piston chamber 1274. Air piston 1264 isadapted to provide desired deflection to shroud 1220. The degree ofdeflection provided by air piston 1264 can be manipulated based onalterations to air piston 1264 geometry, such as, for example: alteringthe thickness of the material comprising the forward rigid member 1271,aft rigid member 1272, and central flex member 1273; altering the radiusof the chamber 1274; altering the ear length of the central flex member1273; and/or altering the centerline of the air piston 1264.

In some embodiments, as illustrated in FIGS. 3A and 3B, central flexmember 1273 comprises a ring 1279 or hoop having a U-shaped crosssection which extends radially outward from forward rigid member 1271and aft rigid member 1272 and adapted to expand, contract, or flexprimarily in an axial direction. In other words, expansion andcontraction of air piston 1264 results in axial movement whilesubstantially maintaining a radial alignment.

In some embodiments high pressure air is received from high pressure airsource 1262 via a receiving chamber 1275 which is in fluid communicationwith piston chamber 1274. In some embodiments receiving chamber 1275includes a regulating valve which regulates movement of high pressureair into and out of piston chamber 1274. In some embodiments receivingchamber 1275 further includes a member for venting piston chamber 1274to atmospheric pressure or to a pressure which is lower than that ofpiston chamber 1274.

Air piston 1264 is axially disposed between a portion of engine casing1231 and shroud 1220. A forward-extending arm 1276 extends axiallyforward from forward rigid portion 1271 and is coupled to engine casing1231 at first mounting flange 1233, thus mounting air piston 1264 to thecasing 1231. An aft-extending arm 1277 extends axially aft from aftrigid portion 1272 and is coupled to a mounting arm 1278 extendingaxially forward from shroud 1220. Aft-extending arm 1277 and mountingarm 1278 are coupled at mounting flange 1237.

In some embodiments air piston 1264 is an annular piston. In otherembodiments, a plurality of discrete air pistons 1264 arecircumferentially disposed about shroud 1220 and each act independentlyupon the shroud 1220.

Shroud 1220 is a dynamically moveable impeller shroud. Shroud 1220encases the plurality of blades 1212 of the centrifugal compressor 1210.Shroud 1220 comprises a forward end portion 1233 terminating at slidablecoupling 1266, a central portion 1224, and a aft end portion 1225. Insome embodiments, surface 1222 of shroud 1220 comprises an abradablesurface. In some embodiments, a replaceable cover is provided whichcovers the surface 1222 and is replaced during engine maintenance due toimpingement of blade tips 1213 against surface 1222.

In some embodiments aft end portion 1225 is defined as the radiallyoutward most third of shroud 1220. In other embodiments aft end portion1225 is defined as the radially outward most quarter of shroud 1220. Instill further embodiments aft end portion 1225 is defined as theradially outward most tenth of shroud 1220. In embodiments whereinmounting arm 1278 extends axially forward from aft end portion 1225,these various definitions of aft end portion 1225 as either the finalthird, quarter, or tenth of shroud 1220 provide for the various radialplacements of mounting arm 1278 relative to shroud 1220.

Slidable coupling 1266 comprises an axial member 1280 coupled to forwardend portion 1233 of shroud 1220. Slidable coupling 1266 is adapted toallow sliding displacement between axial member 1280 and forward endportion 1233. In some embodiments one or more surfaces of forward endportion 1233 and/or axial member 1280 comprise a lubricating surface toreduce friction and wear between these components. In some embodimentsthe lubricating surface is a coating.

Clearance control system 1260 is coupled to the engine casing 1231 via afirst mounting flange 1233 and second mounting flange 1235. In someembodiments engine casing 1231 is at least a portion of a casing aroundthe multi-stage axial compressor.

The gap between a surface 1222 of shroud 1220 which faces the impeller1211 and the impeller blade tips 1213 is the blade tip clearance 1240.In operation, thermal, mechanical, and pressure forces act on thevarious components of the centrifugal compressor system 1200 causingvariation in the blade tip clearance 1240. For most operatingconditions, the blade tip clearance 1240 is larger than desirable forthe most efficient operation of the centrifugal compressor 1210. Theserelatively large clearances 1240 avoid rubbing between blade tip 1213and the surface 1222 of shroud 1220, but also result in high leakagerates of working fluid past the impeller 1211. It is therefore desirableto control the blade tip clearance 1240 over a wide range of steadystate and transient operating conditions. The disclosed clearancecontrol system 1260 provides blade tip clearance 1240 control bypositioning shroud 1220 relative to blade tips 1213.

FIG. 5 is a schematic and sectional view of the pressure regions 1P1,1P2, and 1P3 of a clearance control system 1260 in accordance with someembodiments of the present disclosure. A first pressure region 1P1 isdefined as piston chamber 1274 and receiving member 1275. A secondpressure region 1P2 is disposed radially inward from air piston 1264 andradially outward from shroud 1220 and axial member 1280. A thirdpressure region 1P3 is disposed radially outward from air piston 1264and radially inward from a casing arm 1702.

In some embodiments, second pressure region 1P2 and third pressureregion 1P3 are maintained at or near atmospheric pressure, meaning thatregions 1P2 and 1P3 are neither sealed nor pressurized. First pressureregion 1P1 receives high pressure air from high pressure air source 262,which in some embodiments is compressor discharge air. However, in suchan embodiment, a relatively large piston chamber 1274 is required toovercome the large differential pressure across the shroud 1220 (i.e.differential pressure between the pressure of regions 1P2 and 1P3 andthe pressure of the centrifugal compressor 1210. In other words, thelarge differential pressure makes it more difficult to deflect or causeaxial movement in shroud 1220, thus requiring a larger air piston 1264to perform the work.

Thus in other embodiments second pressure region 1P2 and third pressureregion 1P3 are sealed and pressurized to reduce the differentialpressure across the shroud 1220. For example, in some embodiments secondpressure region 1P2 and third pressure region 1P3 are pressurized usingone of inducer air, exducer air, or intermediate stage compressor air.Supplying compressor discharge air to piston chamber 1274 still createsa differential pressure across the air piston 1264 that causes axialdeflection, but the force required to move shroud 1220 is greatlyreduced due to the lower differential pressure across the shroud 1220.

In embodiments with second pressure region 1P2 sealed and pressurizedusing inducer air and third pressure region 1P3 sealed and pressurizedusing exducer air, the selection of the location of mounting arm 1278between forward end 1233 and aft end 1225 is significant because agreater exposure of shroud 1220 to exducer pressure results in less workrequired by the air piston 1264 to move shroud 1220. In addition, it canbe undesirable to locate mounting arm 1278 adjacent to aft end 1225 dueto the risk that the air piston 1264 will overly bend the upper tip ofshroud 1220.

In some embodiments second pressure region 1P2 and third pressure region1P3 are merged as a single, sealed pressure region and are thuspressurized at equal pressures.

FIG. 3B is an enlarged schematic and sectional view of the clearancecontrol system 1260 illustrated in FIG. 3A, in accordance with someembodiments of the present disclosure. The operation of clearancecontrol system 1260 will be discussed with reference to FIG. 3B.

In some embodiments during operation of centrifugal compressor 1210blade tip clearance 1240 is monitored by periodic or continuousmeasurement of the distance between surface 1222 and blade tips 1213using a sensor or sensors positioned at selected points along the lengthof surface 1222. When clearance 1240 is larger than a predeterminedthreshold, it may be desirable to reduce the clearance 1240 to preventleakage and thus improve centrifugal compressor efficiency. Pressureinside the piston chamber 1274 may be adjusted based on measured bladetip clearance 1240 to move shroud 1220 and thus adjust the blade tipclearance 1240 as desired.

In other embodiments, engine testing may be performed to determine bladetip clearance 1240 for various operating parameters and a piston chamber1274 pressure schedule is developed for different modes of operation.For example, based on clearance 1240 testing, piston chamber 1274pressures may be predetermined for cold engine start-up, warm enginestart-up, steady state operation, and max power operation conditions. Asanother example, a table may be created based on blade tip clearance1240 testing, and piston chamber 1274 pressure is adjusted according tooperating temperatures and pressures of the centrifugal compressor 1210.A sensor may be used to monitor pressure in piston chamber 1274. Thus,based on monitoring the operating conditions of the centrifugalcompressor 1210 such as inlet pressure, discharge pressure, and/orworking fluid temperature, a desired blade tip clearance 1240 isachieved according to a predetermined schedule of pressures for pistonchamber 1274.

Regardless of whether clearance 1240 is actively monitored or controlledvia a schedule, in some operating conditions it may be desirable toreduce the clearance in order to reduce leakage past the centrifugalcompressor 1210. In order to reduce the clearance 1240, high pressuregas is supplied by high pressure gas source 1262 to piston chamber 1274.Piston chamber 1274 expands between forward rigid member 1271 and aftrigid member 1272 due to the admission of high pressure gas. Centralflex member 1273 enables this expansion in an axial direction. With airpiston 1264 rigidly coupled, or “grounded”, to casing 1231 viaforward-extending arm 1276, expansion of the air piston 1264 is enabledin the axially aft direction as indicated by arrow 291 in FIG. 3B.

The axially aft expansion of air piston 1264 displaces aft-extending arm1277 and mounting arm 1278. Mounting arm 1278 is coupled to and impartsa force on the aft end portion 1225 of shroud 1220, thus moving the aftend portion 1225 in an axially aft direction as indicated by arrow 1292.This movement of aft end portion 1225 is translated to a similar axiallyaft movement at the slidable coupling 1266, where forward end portion1233 is displaced in an axially aft direction relative to axial member1280 as indicated by arrow 1293. Additionally, as discussed withreference to FIG. 5, the application of air pressure at third pressureregion 1P3 imparts a force on aft end portion 1225. Shroud 1220 thusmoves relative to the centrifugal compressor 1210 in an axial directionwhile substantially maintaining the radial alignment of shroud 1220.

The axially aft movement of shroud 1220 caused by air piston 1264expansion results in shroud 1220 moving closer to blade tips 1213, thusreducing the clearance 1240 and leakage. During many operatingconditions this deflection of shroud 1220 in the direction of blade tips1213 is desirable to reduce leakage and increase compressor efficiency.

Where monitoring of blade tip clearance 1240 indicates the need for anincrease in the clearance 1240, high pressure air is bled from pistonchamber 1274. As piston chamber 1274 contracts, central flex member 1273enables the contraction to be primarily in the axial direction,resulting in axially forward movement of aft-extending arm 1277,mounting arm 1278, and aft end portion 1225. The axially forwardmovement of aft end portion 1225 results in similar movement of shroud1220, including the sliding displacement in an axially forward directionof forward end portion 1233 against axial member 1280. Thus, by bleedingair from piston chamber 1274 shroud 1220 is moved axially forward, awayfrom blade tips 1213 and increasing blade tip clearance 1240. Slidablecoupling 1266 is dimensioned such that an air boundary is maintainedthrough the full range of axial movement of shroud 1220.

FIG. 4 is a schematic and sectional view of another embodiment of aclearance control system 1360 with a bellows-type air piston 1364 inaccordance with the present disclosure. Air piston 1364 comprises abellows 1379 as central flex member 1273 forming a hoop disposed betweenforward rigid member 1271 and aft rigid member 1272. Like flexibleprotrusion 1279, bellows 1379 is adapted to expand, contract, or flexprimarily in an axial direction. The operation of clearance controlsystem 1360 is substantially the same as the operation of clearancecontrol system 1260 as described above. Bellows 1379 is interchangeablewith flexible protrusion 1279, and central flex member 1273 can takemany forms.

FIG. 6A is a schematic and sectional view of a centrifugal compressorsystem 2200 having a clearance control system 2260 in accordance withsome embodiments of the present disclosure. Centrifugal compressorsystem 2200 comprises centrifugal compressor 2210 and clearance controlsystem 2260.

The centrifugal compressor 2210 comprises an annular impeller 2211having a plurality of centrifugal compressor blades 2212 extendingradially from the impeller 2211. The impeller 2211 is coupled to a discrotor 2214 which is in turn coupled to a shaft 2216. Shaft 2216 isrotatably supported by at least forward and aft shaft bearings (notshown) and may rotate at high speeds. The radially-outward surface ofeach of the compressor blades 2212 constitutes a compressor blade tip2213.

As blade 2212 rotates, it receives working fluid at an inlet pressureand ejects working fluid at a discharge pressure which is higher thanthe inlet pressure. Working fluid (e.g. air in a gas turbine engine) istypically discharged from a multi-stage axial compressor (not shown)prior to entering the centrifugal compressor 2210. Arrows A illustratethe flow of working fluid through the centrifugal compressor 2210.Working fluid enters the centrifugal compressor 2210 from an axiallyforward position 2253 at an inlet pressure. Working fluid exits thecentrifugal compressor 2210 at an axially aft and radially outwardposition 2255 at a discharge pressure which is higher than inletpressure.

Working fluid exiting the centrifugal compressor 2210 passes through adiffusing region 2250 and then through a deswirl cascade 2252 prior toentering a combustion chamber (not shown). In the combustion chamber,the high pressure working fluid is mixed with fuel and ignited, creatingcombustion gases that flow through a turbine (not shown) for workextraction.

In one embodiment, the clearance control system 2260 comprises anactuator 299 and an annular shroud 2220. Clearance control system 2260can also be referred to as a compressor shroud assembly. In someembodiments, actuator 299 may comprise an air source 2262, a thermaldriver 2289, and at least one linkage assembly 2288.

Air source 2262 provides air to thermal driver cavity 2286. In someembodiments air source 2262 receives air from more than one location anduses a multi-source regulator valve or mixing valve to send air of anappropriate temperature to thermal driver cavity 2286. For example, insome embodiments air source 2262 receives relatively cool air fromearlier compressor stages and relatively warm air from the discharge ofcentrifugal compressor 2210. When cooling air is desired to be appliedto thermal driver cavity 2286, as explained below, air source 2262 sendsthe relatively cool air received from earlier compressor stages. Whenheating air is desired to be applied to thermal driver cavity 2286, asexplained below, air source 2262 sends the relatively warm air receivedfrom centrifugal compressor 2210 discharge.

Potential sources of cooling air include ambient air, low pressurecompressor discharge air, inter-stage compressor air, and cooling coilor heat exchanger air. Potential sources of warming air includedischarge air of the centrifugal compressor 2210, core engine air,inter-stage turbine air, cooling coil or heat exchanger air,electrically-powered heating coil air, and engine exhaust. In someembodiments warming and/or cooling air flow is replaced by fluid flowsuch as the flow of a lubricating fluid to provide an actuatingtemperature to thermal driver 2289.

In some embodiments air source 2262 receives air from multiple sourcesand mixes them to achieve a desired temperature prior to applying theair to thermal driver cavity 2286.

Thermal driver 2289 comprises an annular ring 2285 and annular seal 2295which together define thermal driver cavity 2286. In some embodimentsthermal driver 2289 further comprises a thermal feed air tube 2294.Annular ring 2285 is formed from a thermally-responsive material suchthat excitement by application of relatively cool or relatively warm aircauses contraction or expansion, respectively. In other words, thermaldriver 2289 radially expands or contracts when exposed to an actuatingtemperature. In some embodiments, annular ring 2285 has a U-shapedradial cross section. In some embodiments, annular ring 2285 and annularseal 2295 comprise a single annular tube, having one or more thermalfeed air tubes 2294 coupled thereto.

Annular seal 2295 is coupled to annular ring 2285 to form an annularthermal driver cavity 2286. This cavity 2286 is in fluid communicationwith the interior 2270 of at least one thermal feed air tube 2294. Insome embodiments, more than one thermal feed air tube 2294 are disposedcircumferentially around the annular ring 2285 and fluidly communicatewith the annular thermal driver cavity 2286. In some embodiments one ormore sensors may be disposed in or in fluid communication with cavity2286 to measure the fluid temperature or fluid pressure of cavity 2286.Thermal driver 2289 may be exposed to warmer or cooler actuatingtemperatures based on the measured fluid temperature or fluid pressureof cavity 2286.

Linkage assembly 2288 comprises a forward linkage 2281, forwardtranslator 2282, aft translator 2283, and aft linkage 2284. Forwardlinkage 2281 and forward translator 2282 are coupled between a forwardcasing member 2287 and thermal driver 2289. Forward linkage 2281 ispivotally mounted to the forward casing member 2287. Aft translator 2283and aft linkage 2284 are coupled between thermal driver 2289 and shroud2220. Aft linkage 2284 is pivotally mounted to the shroud 2220. In someembodiments, a central linkage comprises forward translator 2282, afttranslator 2283, and thermal driver 2289. In some embodiments, more orfewer linkages are used in linkage assembly 2288.

Each of forward linkage 2281 and aft linkage 2284 comprise a pair ofpins 2296 and a linkage member 2297. Each pin 2296 passes through boththe respective linkage member 2297 and respective component which isbeing coupled to the linkage member 2297. For example, pin 2296A passesthrough the linkage member 2297 of forward linkage 2281 and through anaxial extension 2298 of forward casing member 2287, thus forming a pinjoint or hinge between forward casing member 2287 and forward linkage2281. Similar pin joints are formed between forward linkage 2281 andforward translator 2282 (by pin 2296B), between aft translator 2283 andaft linkage 2284 (by pin 2296C), and between aft linkage 2284 and anaxial protrusion 2300 from shroud 2220.

Forward translator 2282 and aft translator 2283 are coupled to annularring 2285 of the thermal driver 2289. Thus, the thermal contraction andexpansion of annular ring 2285, caused by the application of relativelycool or relatively warm air to the thermal driver cavity 2286, causesrelative motion of forward translator 2282 and aft translator 2283.Linkage assembly 2288 is configured to provide axial movement to shroud2220 based on the thermal expansion and contraction of the thermaldriver 2289, while providing little to no radial or circumferentialmovement to shroud 2220.

Forward casing arm 2287 is coupled to a portion of engine casing 2231 atfirst mounting flange 2233. In some embodiments, the portion of enginecasing 2231 is the compressor casing of a multi-stage axial compressordisposed forward of centrifugal compressor 2210.

In some embodiments linkage assembly 2288 is annular. In otherembodiments, a plurality of discrete linkage assemblies 2288 arecircumferentially disposed about shroud 2220 and each act independentlyupon the shroud 2220.

In some embodiments, a thermal actuator 2261 comprises an annular ring2285 and annular seal 2295 which together define thermal driver cavity2286 and at least one linkage assembly 2288. In some embodiments thermalactuator 2261 may further comprise at least one thermal feed air tube2294. In some embodiments, at least three linkage assemblies 2288 may bespaced around the circumference of shroud 2220. In some embodiments, atleast three linkage assemblies 2288 may be spaced around thecircumference of casing 2231.

Shroud 2220 is a dynamically moveable impeller shroud. Shroud 2220encases the plurality of blades 2212 of the centrifugal compressor 2210.Shroud 2220 comprises a forward end portion 2223 terminating at slidingjoint 2266, a central portion 224, and a aft end portion 2225.

In some embodiments aft end portion 2225 is defined as the radiallyoutward most third of shroud 2220. In other embodiments aft end portion2225 is defined as the radially outward most quarter of shroud 2220. Instill further embodiments aft end portion 2225 is defined as theradially outward most tenth of shroud 2220. In embodiments wherein axialprotrusion 2300 extends axially forward from aft end portion 2225, thesevarious definitions of aft end portion 2225 as either the final third,quarter, or tenth of shroud 2220 provide for the various radialplacements of axial protrusion 2300 relative to shroud 2220.

Sliding joint 2266 comprises forward casing arm 2287 coupled to forwardend portion 2223 of shroud 2220. Sliding joint 2266 is adapted to allowsliding displacement between casing arm 2287 and forward end portion2223. In some embodiments one or more surfaces of forward end portion2223 and/or casing arm 2287 comprise a lubricating surface to encouragesliding displacement between these components. In some embodiments thelubricating surface is a coating.

The gap between a surface 2222 of shroud 2220 which faces the impeller2211 and the impeller blade tips 2213 is the blade tip clearance 2240.In operation, thermal, mechanical, and pressure forces act on thevarious components of the centrifugal compressor system 2200 causingvariation in the blade tip clearance 2240. For most operatingconditions, the blade tip clearance 2240 is larger than desirable forthe most efficient operation of the centrifugal compressor 2210. Theserelatively large clearances 2240 avoid rubbing between blade 2212 andthe surface 222 of shroud 2220, but also result in high leakage rates ofworking fluid past the impeller 2211. It is therefore desirable tocontrol the blade tip clearance 2240 over a wide range of steady stateand transient operating conditions. The disclosed clearance controlsystem 2260 provides blade tip clearance 2240 control by positioningshroud 2220 relative to blade tips 2213.

FIG. 6B is an enlarged schematic and sectional view of the clearancecontrol system 2260 illustrated in FIG. 6A, in accordance with someembodiments of the present disclosure. The operation of clearancecontrol system 2260 will be discussed with reference to FIG. 6B.

In some embodiments during operation of centrifugal compressor 2210blade tip clearance 2240 is monitored by periodic or continuousmeasurement of the distance between surface 2222 and blade tips 2213using a sensor or sensors positioned at selected points along the lengthof surface 2222. When clearance 2240 is larger than a predeterminedthreshold, it may be desirable to reduce the clearance 2240 to preventleakage and thus improve centrifugal compressor efficiency. Actuatingtemperature of thermal driver 2286 may be adjusted based on the measuredblade tip clearance 2240.

In other embodiments, engine testing may be performed to determine bladetip clearance 2240 for various operating parameters and a piston chamber2274 pressure schedule is developed for different modes of operation.For example, based on clearance 2240 testing, piston chamber 2274pressures may be predetermined for cold engine start-up, warm enginestart-up, steady state operation, and max power operation conditions. Asanother example, a table may be created based on blade tip clearance2240 testing, and piston chamber 2274 pressure is adjusted according tooperating temperatures and pressures of the centrifugal compressor 2210.Thus, based on monitoring the operating conditions of the centrifugalcompressor 2210 such as inlet pressure, discharge pressure, and/orworking fluid temperature, a desired blade tip clearance 2240 isachieved according to a predetermined schedule of pressures for pistonchamber 2274.

Regardless of whether clearance 2240 is actively monitored or controlledvia a schedule, in some operating conditions it may be desirable toreduce the clearance 2240 in order to reduce leakage past thecentrifugal compressor 2210. In order to reduce the clearance 2240,relatively cool air is supplied from air source 2262 to thermal drivercavity 2286 via thermal feed air tube 2294. As relatively cool air fillsthe annular thermal driver cavity 2286 it causes contraction of annularring 2285. This contraction reduces the circumference of the ring 2285,such that radially inner surface 2244 moves in a radially inwarddirection as indicated by arrow 2291.

Forward translator 2282 and aft translator 2283 are coupled to ring 2285and therefore also move in a radially inward direction. This radiallyinward motion causes an elongation of linkage assembly 2288, as forwardlinkage 2281 and aft linkage 2284 are pushed by forward translator 2282and aft translator 2283, respectively, in a radially inward direction.The pin joints created by pins 2296A, 2296B, 2296C, and 2296D cause thisradially inward motion to be translated to axial motion.

With forward linkage 2281 coupled to forward casing arm 2287, which isin turn rigidly coupled, or “grounded”, to casing 2231 via mountingflange 2233, motion in the axially forward direction is prohibited.Thus, linkage assembly 2288 translates the radially inward motion ofring 2285 into an axially aft motion.

Aft linkage 2284 acts on axial protrusion 2300, causing aft end portion2225 of shroud 2220 to move in an axially aft direction as indicated byarrow 2292. This movement of aft end portion 2225 is translated to asimilar axially aft movement at the sliding joint 2266, where forwardend portion 2223 is displaced in an axially aft direction relative toforward casing arm 2287 as indicated by arrow 2293. In other words,expansion and contraction of annular ring 2285 results in axial movementof shroud 2220 while substantially maintaining a radial alignment.

The axially aft movement of shroud 2220 caused by ring 2285 contractionresults in shroud 2220 moving closer to blade tips 2213, thus reducingthe clearance 2240 and leakage. During many operating conditions thisdeflection of shroud 2220 in the direction of blade tips 2213 isdesirable to reduce leakage and increase compressor efficiency.

Where monitoring of blade tip clearance 2240 indicates the need for anincrease in the clearance 2240, the process described above is reversed.Relatively warmer air is supplied from air source 2262 to thermal drivercavity 2286, causing expansion of ring 2285. This expansion results in aradially outward movement of ring 2285, forward translator 2282, and afttranslator 2283, which is in turn translated to an axially forwardmotion by linkage assembly 2288. Aft end portion 2225 is pulled bylinkage assembly 2288 in an axially forward direction, and shroud 2220moves in an axially forward direction accordingly. Sliding displacementat sliding joint 2266 allows forward end portion 2223 to move axiallyforward relative to forward casing arm 2287. Thus, by applyingrelatively warmer air to thermal driver cavity 2286, shroud 2220 ismoved axially forward away from blade tips 2213, increasing blade tipclearance 2240. Slidable coupling 2266 is dimensioned such that an airboundary is maintained through the full range of axial movement ofshroud 2220.

FIG. 7 is a schematic and sectional view of another embodiment of aclearance control system 2360 in accordance with the present disclosure.In the embodiment of FIG. 7, axial protrusion 2300 extends from shroud2220 at central portion 224 as opposed to aft end portion 2225.

In some embodiments central portion 2224 is defined as the centermostthird of shroud 2220. In other embodiments central portion 2224 isdefined as the centermost quarter of shroud 2220. In still furtherembodiments central portion 2224 is defined as the centermost tenth ofshroud 2220. In embodiments wherein axial protrusion 2300 extendsaxially forward from central portion 2224, these various definitions ofcentral portion 2224 as either the centermost third, quarter, or tenthof shroud 2220 provide for the various radial placements of axialprotrusion 2300 relative to shroud 2220.

Although the embodiment of FIG. 7 operates in substantially the samemanner as the clearance control system 2260 of FIGS. 6A and 6B, asdescribed above, it should be noted that in the embodiment of FIG. 7 theshroud 2220 is subject to less flexion force due to the centralplacement of axial protrusion 2300 and its connection to linkageassembly 2288. In other words, moving the axial protrusion 2300 morecentrally vice at the aft end portion 2225 results in axially aftdirectional force being applied at central portion 2224 and less flexingof the shroud 2220.

FIG. 8 is a schematic and sectional view of the pressure regions P1, P2,and P3 of a clearance control system 2260 in accordance with someembodiments of the present disclosure. A first pressure region P1 isdefined as thermal driver cavity 2286 and the interior of thermal feedair tube 2294. A second pressure region P2 is defined between shroud2220, forward casing arm 2287, and outward casing member 2401. A thirdpressure region P3 is disposed axially forward of forward casing arm2287.

In some embodiments, second pressure region P2 is maintained at or nearatmospheric pressure, meaning that region P2 is neither sealed norpressurized. However, relatively low pressures in region P2 creates alarge differential pressure across shroud 2220 (i.e. differentialpressure between the pressure of region P2 and the pressure of thecentrifugal compressor 2210) such that it is more difficult to deflector cause axial movement in shroud 2220.

In other embodiments second pressure region P2 is sealed and pressurizedto reduce the differential pressure across the shroud 2220. For example,in some embodiments second pressure region P2 is pressurized using oneof inducer air, exducer air, intermediate stage compressor air, ordischarge air from the centrifugal compressor 2210. The force requiredto move shroud 2220 is greatly reduced due to the lower differentialpressure across the shroud 2220.

In some embodiments third pressure region P3 is pressurized with inducerair and is therefore at a lower pressure than second pressure region P2.

FIG. 9 is a schematic and sectional view of another embodiment of aclearance control system 2760 in accordance with the present disclosure.Clearance control system 2760 comprises an air source 2262, a thermaldrive assembly 2263, and an annular shroud 2220.

Air source 2262 and annular shroud 2220 are substantially the same, andoperates in substantially the same manner, as discussed above withreference to FIGS. 6A and 6B.

Thermal drive assembly 2263 comprises an annular thermal drive ring2265, a drive ring sleeve 2267, and thermal feed air tube 2294. Thermaldrive ring 2265 is coupled between a portion of the engine casing 2231at mounting flange 2233 and a mount platform 268 extending axiallyforward from the aft end portion 2225 of shroud 2220. Thermal drive ring2265 is formed from a thermally-responsive material such that excitementby application of relatively cool or relatively warm air causescontraction or expansion, respectively. Thermal drive ring 2265 is sizedto meet the actuation needs of clearance control system 2760.

Drive ring sleeve 2267 is coupled to thermal drive ring 2265 to form anannular cavity 2269. This cavity 2269 is in fluid communication with theinterior 2270 of at least one thermal feed air tube 2294. In someembodiments, more than one thermal feed air tube 2294 are disposedcircumferentially around the thermal drive ring 2265 and fluidlycommunicate with the annular cavity 2269.

Regardless of whether clearance 2240 is actively monitored or controlledvia a schedule, in some operating conditions it will be desirable toreduce the clearance 2240 in order to reduce leakage past thecentrifugal compressor 2210. In order to reduce the clearance 2240,relatively warm air is supplied from air source 2262 to annular cavity269 via thermal feed air tube 2294. As relatively warm air fills theannular cavity 2269 it causes expansion, primarily in the axialdirection, of thermal drive ring 2265. This axial expansion is anchored,or “grounded”, against the engine casing 2231 such that axial expansionor movement is prohibited in the axially forward direction. Thus, theaxial expansion of thermal drive ring 2265 acts in the axially aftdirection as illustrated by arrow 2291, imparting a force on the mountplatform 2268 and thus on the aft end portion 2225 of shroud 2220 asillustrated by arrow 2292. This movement of aft end portion 2225 istranslated to a similar axially aft movement at the sliding joint 2266,where forward end portion 2223 is displaced in an axially aft directionrelative to forward casing arm 2287 as indicated by arrow 2293.

The axially aft movement of shroud 2220 caused by expansion of ring 2265results in shroud 2220 moving closer to blade tips 2213, thus reducingthe clearance 2240 and leakage. During many operating conditions thisdeflection of shroud 2220 in the direction of blade tips 2213 isdesirable to reduce leakage and increase compressor efficiency.

Where monitoring of blade tip clearance 2240 indicates the need for anincrease in the clearance 2240, the process described above is reversed.Relatively cooler air is supplied from air source 2262 to annular cavity2269, causing contraction of ring 2265. This contraction is primarily inthe axial direction and results in the axially forward movement of ring2265 and mount platform 2268. Aft end portion 2225 is pulled in anaxially forward direction, and shroud 2220 moves in an axially forwarddirection accordingly. Sliding displacement at sliding joint 2266 allowsforward end portion 2223 to move axially forward relative to forwardcasing arm 2287. Thus, by applying relatively cooler air to annularcavity 2269, shroud 2220 is moved axially forward away from blade tips2213, increasing blade tip clearance 2240.

In some embodiments alternative clearance control system 2760 has amodified placement of the linkage assembly to shroud connection, similarto the embodiment disclosed with reference to FIG. 7 above.

The present disclosure is further directed to a system which translatesa pivoting motion of a driving mechanism into axial motion of animpeller shroud to control clearance in a centrifugal compressor. FIG.10 is a schematic and sectional view of another embodiment of aclearance control system 3560 in accordance with the present disclosure.Clearance control system 3560 comprises a shroud 3520 threadably coupledto at least one actuator 299 and slidably coupled to at least a portionof a casing 3531, 3532, 3535. In some embodiments shroud 3520 issegregated, while in other embodiments shroud 3520 may be a unitary ornon-segregated component as illustrated in FIG. 10. Actuator 299comprises a threaded member 3563 and driving member 3564 which iscoupled to an actuator ring 3565. Driving member 3564 extends along aradius of and is rotatable about the axis of rotation of the centrifugalcompressor (not shown in FIG. 10). Driving member 3564 is coupled tothreaded member 3563 which comprises a plurality of driving threadsadapted to rotate with said driving member 3564 while maintaining anaxial alignment. Actuator ring 3565 is coupled to a driving mechanism.

Shroud 3520 is carried by various portions of the casing. Shroud 3520 isthreadably coupled at a threaded portion 3528 to threaded member 3563.Threaded portion 3528 comprises a plurality of driven threads. Shroud3520 is coupled to a casing arm 3280 which is slidably coupled to casing3531 and 3532 at slidable junction 3533. Shroud is also slidably coupledaxial casing member 3535 at slidable coupling 3566. Axial casing member3535 is coupled at flange 3536 to casing portion 3534.

When actuator ring 3565 is moved about the axis of the impeller shaft(not shown) (i.e. into or out of the page), driving member 3564 is movedabout the axis of the impeller shaft as well. The motion of drivingmember 3564 is translated by threaded member 3263 as motion in anaxially forward or axially aft direction. Shroud 3520 moves axiallyforward or axially aft, with slidable coupling 3566 allowing axialmotion relative to axial casing member 3535 and slidable junction 3533allowing axial motion relative to casing 3531, 3532. The motion ofshroud 3520 is illustrated using arrows 3591, 3592, and 3593. In otherwords, the motion of driving member 3564 about the axis the impellershaft results in axial movement of shroud 3520 while substantiallymaintaining a radial alignment.

FIG. 11 is a schematic and axial view of a plurality of driving members3564 circumferentially disposed about an impeller shroud 3520 (notshown) in accordance with some embodiments of the present disclosure. Afirst driving mechanism 3302 and second driving mechanism 3304 arecoupled via a first connector 3314 and second connector 3316,respectively, to actuator ring 3565. Driving mechanisms 3302, 3304 causemotion of actuator ring 3565 about an axis parallel to the axis A ofshaft 3216 or about the axis A of shaft 3216 as indicated by arrows 3307and 3309 by moving connectors 3314, 3316 as indicated by arrows 3306,3308.

In some embodiments, more or fewer driving mechanisms are used to impartmotion to actuator ring 3565. For example in some embodiments each ofthe plurality of driving members 3564 may have an individual drivingmechanism. In some embodiments, first driving mechanism 3302 and seconddriving mechanism 3304 may be one of electrical, pneumatic, or hydraulicactuators.

FIG. 11 illustrates a plurality of driving arms 3564 coupled to a singleannular threaded axial member 3563. In some embodiments, a plurality ofdiscrete threaded axial members 3563 are disposed about an annular ring3312 formed by threaded portion 3528 and the axially-extending portionof casing arm 3580. In some embodiments, threaded portion 3528 may be acontinuous annular component; in other embodiments, threaded portion3528 may be a plurality of limited, discrete components.

In the illustrated embodiment, the six driving arms 3564 are coupled toa single actuator ring 3565. In other embodiments, more or fewer drivingarms 3564 may be used. For example, in one embodiment of the presentdisclosure first driving mechanism 3302 is coupled to a single drivingarm 3564 and second driving mechanism 3304 is coupled to a differentsingle driving arm 3564.

In some embodiments, actuator ring 3565 is divided into several portionssuch that a driving mechanism 3302, 3304 controls only a portion of thedriving arms 3564. For example, in some embodiments actuator ring 3565is divided in half such that first driving mechanism 3302 controls halfof the driving arms 3564 and second driving mechanism 3304 controls theother half of the driving arms 3564.

In some embodiments, a clearance control system 3260 may comprise atleast one actuator 299 and a segregated annular impeller shroud 3220that comprises an inducer portion 3223 and exducer portion 3224. FIGS.12A and 12B provide illustrations of such an embodiment. Clearancecontrol system 3260 can also be referred to as a compressor shroudassembly.

As disclosed above, in some embodiments the actuator 299 may comprise apneumatic piston, a set of thermally-actuated linkages, or a threadedmember. In the illustrated embodiment of FIGS. 12A and 12B, actuator 299may comprise a threaded axial member 3263 and driving member 3264.Threaded axial member 3263 is adapted to communicate with a threadedportion 3281 of casing arm 3280. In some embodiments threaded portion3281 may be carried by inducer portion 3223. Driving member 3264 extendsalong a radius of the axis of rotation A of the rotatable centrifugalcompressor 3210 and is coupled to an actuator ring 3265. The movement ofactuator ring 3265 causes driving member 3264 to rotate about an axisparallel to shaft 3216, or the axis of rotation A of shaft 3216, whichin turn causes threaded axial member 3263 to move in an axially forwardor axially aft direction.

Shroud 3220 is partly a dynamically moveable impeller shroud. Segregatedannular impeller shroud 3220 encases the plurality of blades 3212 of thecentrifugal compressor 3210. Shroud 3220 comprises a fixed inducerportion 3223 and a moveable exducer portion 3224.

In some embodiments, inducer portion 3223 is formed as a unitarystructure with casing arm 3280; in other embodiments, inducer portion3223 is formed separate from and coupled to casing arm 3280.

In some embodiments, exducer portion 3224 may be formed as a unitarystructure with threaded axial member 3263; in other embodiments, exducerportion 3224 may be formed separate from and coupled to threaded axialmember 3263. Exducer portion 3224 further comprises a sealing surface3226 which abuts inducer portion 3223. In some embodiments additionalsealing components are utilized to ensure proper sealing between sealingsurface 3226 and inducer portion 3223.

Clearance control system 3260 is coupled to the engine casing via casingarm 3280, which is joined to a first casing portion 3231 and secondcasing portion 3232 at a first mounting flange 3233. In some embodimentsfirst casing portion 3231 is at least a portion of a casing around themulti-stage axial compressor.

In order to reduce the clearance 3240, a driving mechanism 3302(discussed above with reference to FIG. 11) imparts motion to actuatorring 3265. In FIGS. 12A and 12B, the motion of actuator ring 3265 isinto or out of the page about an axis parallel to the axis A of shaft3216 or about the axis A of shaft 3216. This motion of actuator ring3265 results in motion of driving member 3264 about an axis parallel tothe axis A of shaft 3216 or about the axis A of shaft 3216. The motionof driving member 3264 is translated by threaded axial member 3263 asmotion in an axially forward or axially aft direction. With threadedportion 3281 rigidly coupled, or “grounded”, to casing 3231 via casingarm 3280, axial motion is transferred to the exducer portion 3225 ofshroud 3220 as indicated by arrow 3291. In some embodiments, exducerportion 3225 rotates with driving member 3264 as it translates axiallyforward or axially aft.

The present disclosure provides many advantages over previous systemsand methods of controlling blade tip clearances. The disclosed clearancecontrol systems allow for tightly controlling blade tip clearances,which are a key driver of overall compressor efficiency. Improvedcompressor efficiency results in lower fuel consumption of the engine.Additionally, the present disclosure eliminates the use of complicatedlinkages, significant weight penalties, and/or significant powerrequirements of prior art systems.

In embodiments having an air piston as an actuator, utilizing compressordischarge as the high pressure gas source obviates the need to attach anactuator external to the compressor or engine. The use of an air pistonprovides for manufacturing the shroud from a rigid or primarily rigidmaterial, with the piston chamber supplying axial deflection of theshroud.

In embodiments having a thermal actuator, the use of thermal gradientsin the engine as an actuator for the impeller shroud additionallyeliminates the need for an actuator external to the engine.

Although examples are illustrated and described herein, embodiments arenevertheless not limited to the details shown, since variousmodifications and structural changes may be made therein by those ofordinary skill within the scope and range of equivalents of the claims.

1-17. (canceled)
 18. A compressor shroud assembly in a turbine engine,the compressor shroud assembly comprising: a static compressor casing;an impeller shroud for encasing a rotatable centrifugal compressor, theimpeller shroud extending from an aft end to a forward end; and anactuator mounted between the casing and the impeller shroud, theactuator having an aft-extending arm coupled to the impeller shroudproximate the aft end of the impeller shroud, wherein the impellershroud moves relative to the rotatable centrifugal compressor in anaxial direction when the actuator is actuated.
 19. The compressor shroudassembly of claim 18, further comprising a chamber bounded in part bythe casing and at least a portion of the impeller shroud proximate theaft end thereof, the chamber being pressurized by exducer air.
 20. Thecompressor shroud assembly of claim 18, further comprising a chamberbounded in part by the casing and at least a portion of said impellershroud proximate the forward end thereof, the chamber being pressurizedby inducer air.
 21. The compressor shroud assembly of claim 18, whereinthe actuator is an air piston comprising an air chamber adapted toreceive actuator air.
 22. The compressor shroud assembly of claim 21,wherein the actuator further includes a forward rigid member mounted ata forward end to the casing, an aft rigid member coupled to an aft endto the aft-extending arm, and a flexible member that extends between andinterconnects the forward and aft rigid members to thereby form the airchamber.
 23. The compressor shroud assembly of claim 22, wherein theflexible member comprises a hoop having a U-shaped cross-section thatextends radially outward from the forward rigid member
 24. Thecompressor shroud assembly of claim 21, wherein the air piston is anannular piston.
 25. The compressor shroud assembly of claim 21, whereinthe air piston is a plurality of discrete air pistons.
 26. Thecompressor shroud assembly of claim 21, wherein the actuator furtherincludes a high pressure air source fluidly coupled to the air chamberof the air piston through a receiving chamber, the receiving chamberincluding a regulating valve configured to control the flow of actuatorair from the high pressure air source into and out of the air chamber.27. The compressor shroud assembly of claim 18, wherein the actuatorfurther includes a forward-extending arm coupled to the casing and theactuator further includes a forward rigid member mounted at a forwardend to the forward-extending arm, an aft rigid member coupled to an aftend to the aft-extending arm, and a flexible member that extends betweenand interconnects the forward and aft rigid members to thereby form theair chamber.
 28. The compressor shroud assembly of claim 18, wherein theimpeller shroud is coupled at the forward end to the casing by aslidable coupling that maintains an air boundary during the full rangeof axial movement of said impeller shroud.
 29. A compressor shroudassembly in a turbine engine, the compressor shroud assembly comprising:a static compressor casing; an actuator mounted to the casing; a firstarm coupled to the actuator that moves in an axial direction whilemaintaining a radial alignment when the actuator is actuated; and animpeller shroud for encasing a rotatable centrifugal compressor, theimpeller shroud coupled to the first arm proximate an aft end of theimpeller shroud, wherein the impeller shroud moves relative to therotatable centrifugal compressor in the axial direction when theactuator is actuated.
 30. The compressor shroud assembly of claim 29,wherein the actuator is an air piston comprising an air chamber adaptedto receive actuator air.
 31. The compressor shroud assembly of claim 29,wherein the actuator further includes a first rigid member mounted at afirst end to the casing, a second rigid member coupled at a second endto the first arm, and a flexible member that extends between andinterconnects the first and second rigid members to form an air chamber.32. The compressor shroud assembly of claim 31, wherein the flexiblemember comprises a hoop having a U-shaped cross-section that extendsradially outward from the first rigid member.
 33. The compressor shroudassembly of claim 29, further comprising a second arm that extendsbetween the actuator and the casing to couple the actuator to thecasing.
 34. The compressor shroud assembly of claim 33, wherein theactuator further includes a first rigid member mounted at a first end tothe first arm, a second rigid member coupled at a second end to thesecond arm, and a flexible member that extends between and interconnectsthe first and second rigid members.
 35. The compressor shroud assemblyof claim 34, wherein the flexible member comprises a hoop having aU-shaped cross-section that extends radially outward from the firstrigid member.
 36. The compressor shroud assembly of claim 29, whereinthe impeller shroud is coupled at a forward end to the casing by aslidable coupling that maintains an air boundary during the full rangeof axial movement of said impeller shroud.
 37. The compressor shroudassembly of claim 29, further comprising a chamber bounded in part bythe casing and at least a portion of the impeller shroud proximate theaft end thereof, the chamber being pressurized by one of inducer air,exducer air, and intermediate stage compressor air.